To date, most transistors used in power electronic applications have typically been fabricated with silicon (Si) semiconductor materials. Common transistor devices for power applications include Si CoolMOS, Si Power MOSFETs, and Si Insulated Gate Bipolar Transistors (IGBTs). While Si power devices are inexpensive, they suffer from a number of disadvantages, including relatively low switching speeds and high levels of electrical noise. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-N semiconductor devices, such as gallium nitride (GaN) devices, are now emerging as attractive candidates to carry large currents, support high voltages and to provide very low on-resistance and fast switching times. While numerous III-N transistors and diodes have been demonstrated, improvements in reliability are still necessary in order to enable large scale manufacturing and more widespread adoption of these devices.
FIG. 1 shows a transistor of the prior art having source electrode 14, drain electrode 15, gate electrode 13 and access regions 23 and 24. As used herein, the “access regions” of a transistor refer to the two regions between the source and gate electrodes, and between the gate and drain electrodes of the transistor, i.e., regions 23 and 24, respectively, in FIG. 1. Region 23, the access region on the source side of the gate, is typically referred to as the source access region, and region 24, the access region on the drain side of the gate, is typically referred to as the drain access region. As used herein, the “gate region” 31 of a transistor refers to the portion of the transistor between the two access regions 23 and 24 in FIG. 1.
In typical power switching applications in which switching transistors are used, the transistor is at all times in one of two states. In the first state, which is commonly referred to as the “on state”, the voltage at the gate electrode relative to the source electrode is higher than the transistor threshold voltage, and substantial current flows through the transistor. In this state, the voltage difference between the source and drain is typically low, usually no more than a few volts, such as about 0.1-5 volts. In the second state, which is commonly referred to as the “off state”, the voltage at the gate electrode relative to the source electrode is lower than the transistor threshold voltage, and no substantial current flows through the transistor. Whether the device is on or off depends on whether or not current is able to flow through the device (off=no current). Current flow is determined by the voltage on the gate. When the device is on (gate voltage is high), only a small voltage (0.1-5V) at the drain is required to keep the current flowing. Whereas, when the device is off (gate voltage is low), no substantial current flows regardless of how much voltage is applied to the drain (up to the high voltage limit of the device, at which point the device breaks down).
In the off state, the voltage between the source and drain can range anywhere from about 0V to the value of the circuit high voltage supply, which in some cases can be as high as 100V, 300V, 600V, 1200V, 1700V, or higher. When the transistor is in the off state, it is said to be “blocking a voltage” between the source and drain. As used herein, the term “blocking a voltage” refers to the ability of a transistor to prevent substantial current, such as a current that is greater than 0.001 times the operating current during regular conduction, from flowing through the transistor when a voltage is applied across the transistor. In other words, while a transistor is blocking a voltage applied across it, the total current passing through the transistor will not be greater than 0.001 times the operating current during regular conduction.
As used herein, a “high-voltage device”, such as a high-voltage transistor, is an electronic device which is optimized for high-voltage switching applications. That is, when the transistor is off, it is capable of blocking high voltages, such as about 300V or higher, about 600V or higher, about 1200V or higher, or about 1700V or higher, and when the transistor is on, it has a sufficiently low on-resistance (RON) for the application in which it is used, i.e., it experiences sufficiently low conduction loss when a substantial current passes through the device. A high-voltage device can at least be capable of blocking a voltage equal to the high-voltage supply or the maximum voltage in the circuit for which it is used. A high-voltage device may be capable of blocking 300V, 600V, 1200V, 1700V, or other suitable blocking voltage required by the application. In other words, a high-voltage device can block any voltage between 0V and at least Vmax, where Vmax is the maximum voltage that could be supplied by the circuit or power supply. In some implementations, a high-voltage device can block any voltage between 0V and at least 2*Vmax.